NOx gas sensor including nickel oxide

ABSTRACT

One example includes a sensor for sensing NO x , including an electrically insulating substrate, a first electrode and a second electrode, each disposed onto the substrate, wherein each of the first electrode and the second electrode has a first end configured to receive a current and a second end and a sensor element formed of nickel oxide powder, the sensor element disposed on the substrate in electrical communication with the second ends of the first electrode and the second electrode. In some examples, electronics are used to measure the change in electrical resistance of a sensor in association with NOx concentration near the sensor. In some examples, the sensor is maintained at 575° C.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority benefit under 35U.S.C. § 121 to co-pending U.S. patent application Ser. No. 13/407,453,filed Feb. 28, 2012, and entitled “NOX GAS SENSOR INCLUDING NICKELOXIDE”, which claims priority to U.S. Provisional Patent Application No.61/447,351, filed Feb. 28, 2011, and entitled “NOX GAS SENSOR INCLUDINGNICKEL OXIDE”, which is related to U.S. Patent Application PublicationNo, 2008/0020504 and U.S. Patent Application Publication No.2009/0020422, all of which are incorporated by reference herein in theirentirety.

BACKGROUND

The generic term “NOx” describes oxides of nitrogen, such as nitrogenmonoxide (“NO”) and nitrogen dioxide (“NO₂”). NOx is produced as aby-product of combustion in some engines. Aircraft engines, automobileengines, and power generators, for example, utilize combustion processesthat produce NOx.

NOx is believed to contribute to the production of acid rain, smog, andthe depletion of the ozone layer. For example, reactions includingvolatile organic compounds and NOx, occurring in sunlight, are believedto form ozone. Ground-level ozone is believed to contribute to throatirritation, congestion, chest pains, nausea and labored breathing. Suchozone is believed to aggravate respiratory conditions like chronic lungand heart diseases, allergies and asthma. Ozone is believed toeffectively “age” lungs and may contribute to lung disease. With anincrease in the number of vehicles, the amount of NOx produced isincreasing, and is believed to exacerbate environmental harm.

For at least these reasons, it is desirable to monitor NOx gasconcentrations in emissions. In fact, NOx sensing, such as for emissioncontrol, is a requirement of regulations governing the operation of somediesel and spark ignition engines. Regions applying such regulationsinclude countries in North America and Europe. As an example, in 2010,the total production of emission-regulated heavy-duty trucks, which willbe required to monitor NOx, will be at least 1.5 million. Similaremission standards are being proposed for other internal combustionengines (i.e., passenger cars, boats, sport vehicles, etc.).Accordingly, there is a need for a reliable NOx sensor to monitor andcontrol emissions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an engine system including a sensor assembly coupledto an exhaust system, according to some examples.

FIG. 2 is a top view of a nitrogen oxide sensor, according to someexamples.

FIG. 3 is a bottom view of a nitrogen oxide sensor, according to someexamples.

FIG. 4 is a side view of a nitrogen oxide sensor, according to someexamples.

FIG. 5A is a partial perspective view of a sensor according to someexamples.

FIG. 5B is a partial perspective view of a sensor according to someexamples.

FIG. 6 is a flow chart of a method for fabricating a nitrogen oxidesensor, according to some examples.

FIG. 7 is a flow chart of a method for synthesizing NiO powder such asnanopowder, according to some examples.

FIG. 8 is a flow chart of a method for mixing a paste including nickeloxide powder such as nanopowder, according to some examples.

FIG. 9 is a flow chart of a method for applying a paste to a substrate,according to some examples.

FIG. 10 is a flow chart of a method for sintering a sensor element,according to some examples.

FIG. 11 shows the performance evaluation of the sensor by measuring theresistance change with gas concentration, and is made up of a nickeloxide sensor example, including nickel oxide powder such as nanopowder,exposed to cocktail gases at 575° C. showing sensitivity to NO and NO₂according to some examples.

FIG. 12 shows the performance evaluation of the sensor by measuring theresistance change with gas concentration, and the engine response atdifferent operational steps and sensor sensitivity to NOx, according tosome examples.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments that are practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the embodiments, and it is to be understood that otherembodiments are utilized and that structural, logical and electricalchanges are made. The following description of example embodiments is,therefore, not to be taken in a limited sense, with the scope beingdefined by the appended claims.

NOx is sometimes estimated from measured NO, based on an assumption thattotal NOx is 5% NO₂ and 95% NO. This assumption is generally acceptablewhen combustion exhaust gases are measured at the outlet of thecombustion system, and the oxygen concentration is low. If themeasurement is made at the exhaust outlet or in the atmosphere, however,the NO₂ percentage is likely higher than 5% of the total NOx. For atleast this reason, measurement of both NO and NO₂ is recommended foraccurate total NOx concentration. Accordingly, the present subjectmatter provides sensors that measure both NO and NO₂, rather thanderiving one measurement from the other.

The present subject matter provides several advantages. The sensorsdisclosed herein provide exemplary syntheses of NiO powder such asnanopowder capable of producing increasing conductivity (i.e.,decreasing resistance measured at a sensor) in the presence of both NOand NO₂ gases. Various examples provide equal sensitivity to NO and NO₂,hence, reliable measurement of the total NOx content irrespective of theNO and NO₂ ratio. Examples disclosed herein provide fast response andrecovery time (e.g., times less than 2 seconds for T66). Examplesdisclosed herein provide very low cross sensitivity (e.g., <2% of NOxsensitivity) to other gases present in the exhaust including, but notlimited to, CO, CO₂, hydrocarbons, etc. Some examples providenanomaterial that is nonreactive to poisonous gases present in theengine exhaust like SO₂ and silicone. Examples disclosed herein arerobust and operate at high temperatures (e.g., at temperatures of around550° C. to 575° C.). Some examples are packaged and are protected fromcontaminants like soot and hard particles present in the engine exhaust.Some examples are compatible with use directly in the engine exhauststream. Various examples function stably in the various humidity levels(10% to 90%) in an atmosphere.

Certain examples include nickel oxide (“NiO”) powder such as nanopowdersynthesis showing change in conductivity to reducing NO gas andoxidizing NO₂ gas. Some examples demonstrate equal sensitivity to NO andNO₂ at an elevated working temperature of 550° C. to 575° C. Someexamples are stable in corrosive and harsh environments such as thoseincluding SO₂, water vapor, high concentrations of CO₂, and combinationsthereof.

FIG. 1 illustrates an engine system including a sensor assembly 108coupled to an exhaust system 106, according to various examples. Theengine system includes a combustion engine 102, such as a diesel engine,coupled to an intake system 104 and an exhaust system 106 to dispose ofan exhaust stream of the combustion engine. A sensor assembly 108 iscoupled to the exhaust system 106.

The sensor assembly 108 includes a NOx sensor 114 to sense theconcentration of NOx in the exhaust stream of the exhaust system 106.Information associated with the changing NOx level is to inform one ormore computers or engine operators how the engine is operating duringengine calibration or engine operation. In some examples, NOxinformation is used as an input to one or more automatic controls usedby an interconnected device, such as an engine controller, to controlcombustion. To monitor exhaust, some examples use a NOx sensing circuit110 coupled to the sensor assembly 108 and NOx sensor 114 to detect aNOx indication produced by the NOx sensor 114 while the exhaust streampasses nearby the NOx sensor 114.

Although exhaust gases are heated while the system is in operation, insome examples, the sensor assembly 108 includes a heating element 118,controlled by the heating circuit 120, configured to heat the NOx sensor114.

FIGS. 2-4 illustrate, according to some examples, a sensor 10 fordetecting NOx. According to several examples, the sensor 10 comprises asubstrate 12 having a first surface 14 and a second surface 16. In someexamples, the second surface 16 is parallel to first surface 14. Thesubstrate may be formed from any suitable electrically-insulating andheat resistant material such as, for example, a ceramic. In someexamples, the substrate is formed of alumina (Al₂O₃). The substrate 12may have any suitable size and shape. In some examples, the substrate 12includes an elongated plate having a thickness in the range of about 0.5millimeters (mm) to about 1 mm. In some examples, the substrate has athickness of around 0.65 mm.

The sensor 10 includes a first electrode 18 and a second electrode 20(e.g., inter-digital electrodes) disposed on the first surface 14 of thesubstrate 12. The electrodes may be formed of any suitable electricallyconductive material. Examples of suitable materials from which theelectrodes 18 and 20 are formed include, but are not limited to,platinum (Pt), gold (Au), nickel (Ni), silver (Ag), conducting metaloxides, and the like, including combinations thereof. Each of theelectrodes 18 and 20 has a first end 22 and second ends 24. The firstend 22 of each electrode is configured to receive a current. The secondend 24 is configured in any suitable manner for conducting a currenttherebetween. In some examples, the second ends 24 of electrodes 18 and20 are formed in an inter-digital structure, as illustrated in FIG. 2.

An element 26 including nickel oxide (NiO) is disposed in electricalcontact with the second ends 24 of the electrodes 18 and 20. In someexamples, the element 26 is disposed overlying the electrodes 18 and 20.In some examples, the element 26 is formed underlying the electrodes. Insome examples, the second ends 24 of electrodes 18 and 20 are sandwichedbetween two elements 26. In some examples, the element 26 is sandwichedbetween the two electrodes. The element 26 is formed of any suitablenickel oxide material, including powder and nanopowder. In someexamples, the element, is formed of nickel oxide. As illustrated in FIG.4, the element 26 has a thickness, indicated by double-headed arrow 28.In some examples, the thickness 28 is in the range of about 120 microns

In some examples, the sensor 10 comprises a heater 30 disposed on secondsurface 16 of the substrate 12. The heater 30 is comprised of anysuitable heat-conducting material that is capable of heating the element26 to a temperature of at least about 450° C. Possible materials includeplatinum (Pt), gold (Au), nickel (Ni), silver (Ag), conducting metaloxides, and the like, including combinations thereof. In some examples,the heater heats the element 26 to a temperature of at least 500° C. Insome examples, the heater 30 is an elongated conductor formed ofplatinum.

The sensor 10, as described herein, has high sensitivity to NOxconcentrations in a gas. Nickel oxide (NiO) is used as a NOx sensingelement to sense NO and NO₂ or a mixture of NO and NO₂ (NOx), Accordingto some examples, the NiO sensing element 26 acts as a catalyst toconvert NO to NO₂. In some examples, the sensor 10 has improved efficacywhen heated to a working temperature of from about 450° C. to about 575°C.

Without being bound by theory, the sensor experiences the resistancechange at least because of NO₂ adsorption on the surface. In someexamples, NiO powder such as nanopowder acting as a catalyst achievessensitivity for both NO and NO₂ and a change in resistance in the samedirection. In some examples, NiO powder such as nanopowder achieveschanges in resistance for NO and NO₂ in opposite directions. Withoutbeing bound by theory, this is caused at least because NO is a reducinggas and NO₂ is an oxidizing gas. Such a response demonstrates that thesensor element is not converting NO to NO₂, according to some examples.

FIG. 5A is a partial perspective view of a sensor according to someexamples. FIG. 5B is a partial perspective view of a sensor according tosome examples. According to several examples, the sensor 510 comprises asubstrate 512 having a first surface 514 and a second surface, such asopposite the first surface. In some examples, the second surface isparallel to first surface 514. The substrate 512 is formed of anysuitable electrically-insulating and heat resistant material such as,for example, a ceramic. In some examples, the substrate is formed ofalumina (Al₂O₃). The substrate 512 may have any suitable size and shape.In some examples, the substrate 512 includes an elongated plate having athickness in the range of about 0.5 millimeters (mm) to about 1 mm. Insome examples, the substrate has a thickness of around 0.65 mm.

The sensor 510 includes a first electrode and a second electrode (e.g.,inter-digital electrodes) disposed on the first surface 514 of thesubstrate 512. In certain examples, at least a portion of one or both ofthe first and second electrodes is potted with a potting material 532.Some examples include potting, such as with Al₂O₃, and bonding thesensor to wire 534. According to some examples, the sensor is packagedin a housing or housing assembly 536.

In some examples, the sensor 510 comprises a heater, such as a heaterdisposed on a second surface of the substrate, such as a second surfaceopposite the first surface 514. In some examples a wire 538 is coupledto the heater to power the heater.

FIG. 6 illustrates a method 600 for fabricating a sensor for detectingnitrogen oxides. The method starts at 602. At 604, the method includes asynthesis of NiO powder such as nanopowder. FIG. 7 describes an optionalexample of synthesis. At 606, the method includes mixing a pasteincluding NiO powder such as nanopowder and a binder. FIG. 8 describesan optional example of mixing.

At 608, the method includes applying a paste onto a substrate, such asonto a CTL sensor-heater substrate or a Dietrich sensor-heatersubstrate. Some examples include providing an electrically-insulatingand heat-resistant substrate plate having a first surface and a secondsurface. As described herein, the substrate can be formed from anysuitable electrically-insulating and heat-resistant substrate such as,for example, alumina. Two electrodes of an electrically conductivematerial are formed on the first surface of the substrate, according tosome examples. FIG. 8 describes an optional example application ordeposition method in detail. According to some examples, the nickeloxide powder such as nanopowder is deposited as an element onto thefirst surface of the substrate in electrical contact with the secondends of the electrodes.

Some examples comprise forming a heater on the second surface of thesubstrate. In some examples, the heater is formed of the same materialas the electrodes formed on the first surface of the substrate or isformed of any other suitable electrically conductive material such as,for example, platinum (Pt), gold (Au), silver (Ag), nickel (Ni),conducting polymers, conducting metal oxides, and the like, by anysuitable method. In sonic examples, the heater is formed by combining aplatinum paste with a suitable glass matrix and screen-printing theplatinum paste/glass matrix mixture in a desired form onto thesubstrate. In some examples, the heater is then sintered, for example atabout 1000° C. The heater can have any suitable form or structureconducive to heating the element to a temperature from about 400-700° C.Some examples heat to around 575° C.

At 610, the method includes sintering of the paste. As described above,the electrodes can have any suitable form or structure conducive toconducting a current therebetween. In some examples, the electrodes canhave an elongated structure with inter-digital ends, as illustrated inFIG. 1. FIG. 10 describes an optional example sintering method indetail.

At 612, the method includes connecting electrical contacts to thesensor. The electrodes are formed of any suitable electricallyconductive material such as, for example, platinum (Pt), gold (Au),nickel (Ni), silver (Ag), conducting metal oxides, and the like, by anysuitable method. In some examples, the electrodes are formed bycombining a platinum paste, ink, or paint with a suitable glass matrixand screen-printing the platinum paste/glass matrix mixture in a desiredconfiguration onto the substrate.

In one example, the element is deposited overlying the second ends ofthe electrodes. In another example, the element is deposited on thesubstrate before the electrodes are formed on the substrate. In afurther example, an element is deposited before the electrodes areformed on the substrate and is deposited overlying the second ends ofthe electrodes such that the electrodes are effectively “sandwiched”between two elements.

At 614, the method includes packaging the sensor, such as by pottingwith Al₂O₃ and bonding the sensor to wire and packaging the sensor in ahousing. At 616, the process includes an optional method of verifyingthe sensor, such as by testing in a gas formed of N_(2,) 300 PPM CO, 20%CO₂, 200 PPM NH₃, 15 PPM SO₂, and 20% O₂, with the gas at 575° C. At618, the process ends.

FIG. 7 is a flow chart of a method 700 for synthesizing NiO powder suchas nanopowder, according to some examples. At 702, the process starts.At 704, the process includes combining nickel nitrate Ni(NO₃)₂.6H₂Obase, such as in the amount of 25 grams, combined with acetone into mix.The process and apparatus described herein are not limited to nickelnitrate, and other nickel salts, nickel nitrates, nickel acetates,nickel acetyl acetonates, nickel citrates and nickel tartrates both inhydrous and unhydrous form can be used to form the nickel oxide.Examples include, but are not limited to, Ni(NO)₃.6H₂O (Nickel nitratehexahydrate), Ni(NO)₃ (Nickel nitrate unhydrous), Ni(C₂H₃O₂)₂ (Nickelacetate), C₁₂H₁₀Ni₃O₁₄ (Nickel citrate), (C₅H₇O₂)₂Ni.2H₂O (Nickelacetryl acetonate dihydrate), and combinations thereof.

At 706, the method includes grinding the mix, such as for 20 minutes viamortar and pestle. Some examples grind the mix for one hour. At 708, acalcination process begins. At 708 the process includes heating at 120°C. for around 15 minutes. At 710, the method includes grinding thepowder such as nanopowder. At 712, the method includes heating at 250°C. for around 30 minutes. At 714, the method includes grinding thepowder such as nanopowder. At 716, the method includes heating at 650°C. for around 30 minutes. At 718, the method includes grinding thepowder such as nanopowder. At 720, the process ends.

FIG. 8 is a flow chart of a method 800 for mixing a paste includingnickel oxide powder such as nanopowder, according to some examples. At802, the process starts. At 804, the method includes mixing NiO powdersuch as nanopowder with commercial vehicle, such as at 30% commercialvehicle (for example ESL-3032-31) by weight. Examples of commercialvehicles include, but are not limited to, ESL 400, which is aTexanol-based paste product of ESL ElectroScience, King of Prussia, Pa.19406-2625, At 806, the process includes ball milling the mix, such asfor 20 minutes at 450 rpm forward and reverse using 5 mm zirconia (i.e.,zirconium dioxide) balls and zirconia container. An example is a ballmill at 1 hour at 2400 rpm. At 808, the process ends.

FIG. 9 is a flow chart of a method 900 for applying a paste to asubstrate, according to some examples. At 902, the process starts. At904, the process includes maintaining the NiO paste at a selectedviscosity, such as between 180-220 poise. At 906, the process includescreating a NiO film, such as at a thickness of around 120 microns.Creating such a film includes screen-printing, in some examples. In someexamples, the powder such as nanopowder can be dispersed in an organicliquid such as, for example, hexane and deposited on the substrate byspin coating or dip coating or screen printing. It will be appreciatedthat any other suitable method for depositing the element on thesubstrate also is used. At 908, the process ends.

FIG. 10 is a flow chart of a method 1000 for sintering a sensor element,according to some examples. At 1002, the process starts. At 1004, theprocess includes sintering the NiO deposit, such as at 120° C. for 45min in a muffle furnace. At 1006, the process includes sintering the NiOdeposit, such as at 500° C. for 30 min in a muffle furnace. At 1008, theprocess includes sintering the NiO deposit, such as at 700° C. for 5 minin a muffle furnace. At 1010, the method includes cooling the NiOdeposit. At 1012, the process ends.

FIG. 11 is showing the response curves through resistance measurementsof a nickel oxide sensor example, including nickel oxide powder such asnanopowder, exposed to cocktail gases at 575° C., showing sensitivity toNO and NO₂ according to some examples. The response curve shows asquare-wave shaped response, with the high side associated with acocktail of gasses with little or no NO or NO₂, and with the low sideassociated with the presence of one or both NO or NO₂ in an amountdesired to be detected.

FIG. 12 is showing sensor response in engine exhaust at differentoperational steps and sensor sensitivity to NOx, according to someexamples. The step numbers are associated with different engineoperating parameters, including, but not limited to, throttle position,fuel mixture, engine rpm, intake gas composition, and combinationsthereof. In some examples, the step numbers are associated withincrements in RPMs, such as increments of 100 rpm, but the presentsubject matter is not so limited. The response curve shows both that thesensor is able to detect the presence of NOx, and that sensorembodiments of the present disclosure are both accurate and precise.

Accordingly, NOx sensors and methods for forming such sensors have beenprovided. The sensors, according to some examples, are equally sensitiveto NO and NO₂ gases in a gas and are insensitive to CO, O₂, NH₃, andhydrocarbon gases. In addition, the sensors provide fast response andare operable at high temperatures such as approximately 500° C. andhigher.

The Abstract is provided to comply with 37 C.F.R. § 1.72(b) and issubmitted with the understanding that it will not be used to interpretor limit the scope or meaning of the claims.

The invention claimed is:
 1. An apparatus for sensing NOx, comprising:an electrically insulating substrate; a first electrode and a secondelectrode, each disposed onto the substrate, wherein each of the firstelectrode and the second electrode has a first end configured to receivea current and a second end; a sensor element formed of a sintered pastecomprising nickel oxide powder, the sensor element disposed on thesubstrate in electrical communication with the second ends of the firstelectrode and the second electrode, wherein the nickel oxide powdercomprises sintered powder formed of one or more of a group includingnickel salts, organic salts of nickel, and inorganic salts of nickel;and a potting material, wherein at least a portion of the firstelectrode, the second electrode, or both are covered with the pottingmaterial.
 2. The apparatus of claim 1, wherein the sensor element isdisposed on a first surface of the substrate, opposite a second surfaceof the substrate, and further comprising a heater disposed on the secondsurface of the substrate, wherein the heater is configured to heat thesensor element to a temperature of at least about 500° C.
 3. Theapparatus of claim 1, wherein the powder includes a nanopowder.
 4. Theapparatus of claim 1, wherein the potting material comprises Al₂O₃. 5.The apparatus of claim 1, wherein 30% by weight of the sensor elementcomprises a vehicle.
 6. The apparatus of claim 1, wherein the sensingelement is around 120 microns in thickness.
 7. An apparatus comprising:a sensor assembly comprising: a substrate; a first electrode and asecond electrode, each coupled to the substrate, wherein each of thefirst electrode and the second electrode has a first end configured toreceive a current and a second end; and a sensor element comprisingnickel oxide, wherein the sensor element is disposed in electricalcommunication with the second ends of each of the first electrode andthe second electrode, wherein the sensor element is configured toconvert nitric oxide to nitrogen dioxide, wherein the sensor element isnonreactive to sulfur dioxide and silicone; and an engine systemcomprising: a combustion engine; an intake system coupled to thecombustion engine, an exhaust system coupled to the combustion engine,wherein the sensor assembly is coupled to the exhaust system.
 8. Theapparatus of claim 7, wherein the substrate is formed from a ceramic. 9.The apparatus of claim 7, wherein the substrate is formed from alumina.10. The apparatus of claim 7, wherein the first electrode and the secondelectrode are inter-digitated.
 11. The apparatus of claim 7, wherein thefirst electrode and the second electrode are disposed between the sensorelement and the substrate.
 12. The apparatus of claim 7, wherein thesensor element is disposed on the substrate, and wherein the firstelectrode and the second electrode are disposed on the sensor element.13. The apparatus of claim 7, wherein the sensor element is disposed onthe substrate, and wherein the first electrode and the second electrodeare embedded in the sensor element.
 14. The apparatus of claim 7,wherein the sensor element is coupled to a first surface of thesubstrate, opposite a second surface of the substrate, and furthercomprising a. heater disposed on a second surface of the substrate. 15.The apparatus of claim 7, wherein at least one of the first electrode orthe second electrode comprises: platinum (Pt), gold (Au), nickel (Ni),silver (Ag), a conducting metal oxide, or any combination thereof. 16.The apparatus of claim 7, wherein the sensor element is configured toprovide an equal sensitivity to NO and NOx.
 17. The apparatus of claim7, wherein the sensor element has a cross sensitivity to gases otherthan NO and NOx of less than about 2% of a sensitivity to NOx.
 18. Theapparatus of claim 17, wherein the gases comprise CO, CO₂, ahydrocarbon, or any combination thereof.
 19. The apparatus of claim 7,wherein the nickel oxide comprises a sintered nanopowder.